In recent years, non-volatile memory semiconductor devices, in which data is rewritable, have been widely used. For instance, in a flash memory that is a typical non-volatile memory, a transistor composing a memory cell stores charge in a charge storage layer, so that data can be stored. There are two types of flash memories, namely, a floating gate type flash memory having a floating gate as the charge storage layer, and a SONOS (Silicon Oxide Nitride Oxide Silicon) type flash memory having a trap layer as the charge storage layer.
Data can be written into the memory cell (that is, charge is stored in the charge storage layer) by injecting electrons or the like in the charge storage layer. More specifically, a high voltage is applied between the source and drain of the transistor that forms the memory cell, and a positive voltage is applied to the control gate thereof. Hot electrons thus generated between the source and drain are injected into the charge storage layer, and are accumulated therein. The charge (electrons) injected into the charge storage layer raises the threshold voltage of the transistor that forms the memory cell. The threshold voltage is measured in the form of current so that the data stored therein can be read.
As long as a circuit which is an external user device of the flash memory, such as a CPU, is writing data into the flash memory, the CPU can not execute any process other than the data writing process. It is thus demanded to shorten the time it takes to write data into the memory cell in order to improve the CPU process efficiency.
Japanese Patent Application Publication No. 2002-197880 (hereinafter, Document 1) discloses a technique that selectively sets the voltages applied to the source, drain and control gate of the transistors that form the memory cells and the applied times thereof to appropriate values based on applications at the time of writing data into the memory cells.
FIG. 1(a) (Prior Art) shows the time during which data can be retained (data retention time) as a function of data writing depth (that is, threshold voltage), and FIG. 1(b) (Prior Art) shows the time necessary to write data (data writing time) as the function of threshold voltage. The charge stored in the charge storage layer disappears over time (that is, the data is lost). The ordinary flash memories are required to secure long-term data retention as long as 10 to 20 years at a temperature of 90 to 125° C. The long-term data retention may require a larger amount of charge in the charge storage layer. As is shown in FIG. 1(a) (Prior Art), it is required to increase the threshold voltage of the transistors that form the memory cells. However, as is shown in FIG. 1(b) (Prior Art), it takes a longer time to write data in order to store an increased amount of charge in the charge storage layer. At the time of writing data, as the voltage applied to the memory cell is raised, the time necessary for writing data may be shortened. However, the memory cells may be damaged. Further, the threshold voltages may be distributed in a wider range. For these reasons, it is not preferable to increase the voltage applied to the memory cell for data writing. Taking the above into consideration, if the voltage applied to the memory cell is fixed, there is a tradeoff relationship between the elongated data retention time and the reduced data writing time.
The technique described in Document 1 appropriately selects the data retention time and writing time on the basis of applications, and is not capable of securing the sufficient data retention time and reduction in the writing time.